A LAN is a small network within a single building or location.
A WAN links machines in different locations
A simple connection from one machine to another. e.g. dialup link through a modem.
Many machines connected by a single pathway. Only one machine can use the bus at once. Machines must ask to use the bus and may have to wait. e.g. ethernet.
Each machine is connected to a single point.
When a message is to be sent between LANs or over a WAN then it must co through a device called a router. A router may be used to restrict access to machines off the LAN, in this case itis called a firewall.
This diagram shows the Albany Campus Network
For machines to be able to communicate they must use an agreed protocol and messages must travel through different types of hardware between different applications. To understand all this it is easiest to use a layered approach.
The software has four layers
Application Layer : a set of library functions for sending messages etc. e.g. sockets.
Transport Layer: packages up the message for transportation to final destination
Network Layer: packages up the message for transportation to nearest router on route to destination or to local machine.
Link Layer: send the message over the local network.
The following diagram shows how a message is sent between two machines:
The following diagram shows a message passing through two machines acting as routers.
The most common set of network protocols is called TCP/IP (transmission control protocol/ internet protocol)
Application (web browser) uses the application layer to open a connection to the remote machine:
Transport layer breaks message down into packets and adds a destination address and sequence number to each.
Network layer adds address of router to each packet.
Link layer asks for ethernet bus, is given it and puts the message on the bus.
router sees the message on the bus and sends it to another router until a router on the same LAN as the destination is found. This router puts the message on its ethernet bus.
Destination machine link layer sees a message for it on the bus and gives it to the network layer.
Network layer passes message to the transport layer.
Transport layer uses sequence numbers to reconstruct message.
Application layer passes message to waiting application (web server).